Update env_file_reference.md

[BioBit]

Added an explanation how to work with the environmental variables in Portainer.

[recursivetree]

The recommended way to install SeAT doesn't include Portainer. Therefore, I question whether we should have portainer specific steps in the documentation. If we want to include portainer specific things, we should at least explicitly mark it as not part of the official seat stack and preferably move it to a separate guide.

I don't see anything against an explicitly marked portainer guide on the SeAT documentation though.

[BioBit]

I understand. Portainer is pretty much just a managment tool for docker containers using docker and docker compose. So I figured that they are close enough related.

I can draft the sub-chapters for a portainer guide 👍